Electric control circuit



Nov. 1l, 1958 c. J. sPERR, JR 2,860,262

- ELECTRIC CONTROL CIRCUIT Filed March 11, 1955 Cmp, fm

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6 Claims. (Cl. 307-137) This invention relates to an electric control circuit in which the voltage of a source is applied to a load device through two or more sets of switch contacts connected in series with the device. More particularly, the invention relates to such a circuit which is subjected to conditions giving rise to separation of the switch contacts by foreign matter and, consequently, to failure of the circuit to be completed for energization of the load device when desired.

The general object `of the invention is to insure reliable operation of circuits of the above character and avoid the diliculties resulting from accumulation of foreign matter on the switch contacts by a novel arrangement of circuit elements which utilizes the voltage of the source to break down such foreign matter without affecting normal operation of the circuit.

A more detailed object is to provide a novel arrangement of impedance elements for applying across each of the sets of contacts separated by foreign matter substantially all of the voltage of the source or a part thereof suficient to break clown the film.

Other objects and advantages of the invention will become apparent from the following detailed description taken in connection with the accompanying drawings, in which Figures 1, 2, and 3 are schematic wiring diagrams showing different operating conditions of a preferred control circuit embodying the novel features of the present invention.

Fig. 4 is a schematic wiring diagram of a modified circuit.

The present invention is particularly adapted for use in electrical control circuits in which the voltage of a source is applied to a load 11 through the contacts lof two or more switches 12, 13 and 14 connected in series with the load and the source. The load may be any current responsive device such as a relay coil which is energized when the current supplied by the source reached a predetermined level. While the switches may be of different types, each is shown in the present instance as a single pole single throw switch having a movable contact and a Xed contact. Under some operating conditions, circuits of the above character are subject to unreliable operation due to collection on the contacts of one or more of the switches of dust or other foreign matter which prevents closure of the switch and completion of the energizing circuit for the load.

To overcome the foregoing difficulty when operation of two or more of the switches 12, 13 and 14 becomes faulty due to foreign matter on the switch contacts, the present inventio-n contemplates the provision of novel means for selecting the faulty contacts and applying only to such contacts voltage of the source for breaking down the foreign matter. This means comprises impedance elements 15 and 16 which are connected individually in shunt around at least all except one of the switches. Each element has characteristics such that, when the full voltage of the source 10 is applied to the element and the 2,860,262 Patented Nov. l1, 1958 "ice load in series, the current ow in the load is less than the predetermined value necessary for its energization and the voltage drop across the impedance element is equal substantially to the full voltage of the source. The load then remains deenergized and the voltage across the impedance element is available to break down any foreign matter which is separating the contacts of the switch shunted by the element. When two or more impedance elements shunting faulty contacts are connected in series with the load across the source, the load still remains deenergized and the source voltage divides between the elements and the load in proportion to their relative impedance values, the major portion of the source voltage dividing between the impedance elements for application individually to the faulty contacts to break down the foreign matter separating such contacts.

Where the source 10 supplies alternating current as shown in the preferred apparatus of Figs. l to 3, each of the impedance elements 15 and 16 may be a resistor, a capacitor, `or an inductor having an effective impedance equal to several times that of the load 11. In the present instance, the elements are resistors of equal value, these being two resistors one connected in shunt around each of only two of the switches. The source 10 is shown as the secondary winding of a transformer whose primary winding is connected across suitable supply lines (not shown).

Under one operating condition in which the contacts of the unshunted switch 14 are separated only by foreign matter and the shunted switches 12 and 13 are open and closed respectively as shown in Fig. l, no current ows in the circuit and the full voltage of the source is applied across the unshunted contacts through the load, the closed -one 13 of the shunted switches, and the resistor 15 shunting the open switch 12. This voltage then is available to break down the foreign matter on the contacts and permit the latter to close.

Should thevunshunted switch 14 and one shunted switch 12 be closed with the contacts of the other shunted switch 13 separated only by foreign matter as illustrated in Fig. 2, a small current ows in the circuit and the full voltage `of the source is applied across the load 11 and the resistor 16 in series. The voltage across the resistor under this condition is equal substantially to the source voltage due to the relative impedance values of the resistor and the load to break down the foreign matter on the contacts of the faulty switch 13. A similar action occurs if the contacts of the other shunted switch 12 are separated only by foreign matter and the other switches 13 and 14 are closed. If the unshunted switch 14 is closed and the contacts of both shunted switches are separated by foreign matter as shown in Fig. 3, substantially all of the voltage of the source is divided equally between the resistors for application of a part of such voltage individually across each set of faulty contacts.

Where the voltage source 10 supplies direct current as shown in Fig. 4, the impedance elements may be resistors as inthe preferred construction described above or may be capacitors 17 and 18, one connected in shunt around each of two of the three switches 12, 13 and 14. The use of capacitors in a direct current circuit is possible because of the characteristic of a capacitor that it acts as a short circuit when in an uncharged condition. Thus, when all three of the switches are separated by foreign matter as shown in Fig. 4, the capacitors, acting as short circuits around the switches 12 and 13, complete a conductive path from the source and through the load to the unshunted contacts to apply the full voltage of the load to these contacts. Upon closure of the unshunted contacts 14, the source voltage then'is applied to the two capacitors 17 and 18 in series and divides between the latter in inverse proportion to their capacitance values. Where, as in the present instance, the values are equal, the source voltage divides equally between Ythe Ycapacitors Vv17 and 18 for application individually across the open contacts 12 and 13. It will be apparent that the operation of the modified circuit ofl Figf4 vis similarto'that of the prefered'construction under the ydifferent conditions described above.

In one control circuit Aembodying the yinvention and having a load comprising two parallel relay'coils with an energized impedance of approximately 288 ohms connected in series with live sets of switch contacts Vacrossan alternating current source of l-l5 volts, faulty operation of the'switch contacts was avo-ided by connecting a resistor of 10,000 ohms across each of four of the switches. Prior to the use of such shunting resistors, the circuit Vfrequently failedto'operate requiringshutdown'of the ap paratus and cleaning of the Vcontacts of all of the switches,

I'claim as my invention:

1. In combination with a device adapted to be energized by the flow therethrough of electric currentand means including two sets ofswitch contacts connected in series with each other and said device for applying to the latter a voltage of a value Vsuflicient to energize the device when both sets of contacts are closed, a resistor co-nnected in shunt around a first one of said sets of contacts and in series with the second set and having a resistance suiciently higher in value than vthe impedance of said device to limit current flow through the device to a value` insufcient tov energize the Vdevice and to produce a Voltage drop across said rst set of contacts substantially equal to the voltage of said source when said first contacts are separated by foreign matter and said second 'contacts are closed for application of said voltage to the device and said resistor in series, said resistor completing a conductive path through said Adevice for application of .sa1d voltage across said second contacts when the contacts of both of said sets Vare held in open condition by foreign matter thereon.

2. In combination with a device adapted to be energized by the flow therethrough of electric current of a predetermined value and means including two sets 0f switch contacts connected in series with each other `and said device for applying to the device a voltage of a value suffic1ent to energize the same at said current value when both seits of contacts are closed, two capacitors one connected 1n a shunt around each of said sets of contacts and each having an impedance sufficiently higher :in valueA than that of said device to limit current flow thro-ugh thetdevice to a value less than saidpredetermined .value vandtoproduce'a-cross the contacts shunted by the capacitor a voltage drop substantially equal to the voltage of said :source when such shunted contacts are separated by ,foreign matter and the-other contacts are closed, substantially all of said applied voltage dividing between said capacitors for application 'of the voltageacross each capacitor iindividuallyto its shunted contacts to breakdownthe. foreign matter thereon when the contacts of both sets are' separated by such matter.

3. In combination with agdevice adapted to :be lenergized by the llow therethrough of electric currentofa predetermined value and means includingtwo YSets of switchy contactsfc'onnected-in series AwithV eachiother 'and vsaid device for applying to the device a voltage of-a value suicientr-toenergize'the sameat said currentlvalue when? both sets of contacts are closed,facapacitor l"connected in a shunts'around oneofisaid sets of contacts-and having an impedance to current ow correlated W'ithsuch limpedance of said device to limit current ow through the latter to a value less than said predetermined value 'andV to' produce Vacross the contacts shunted'by the capacitor a voltage drop substantially equal to'the voltage of said'source when such shuntedcontacts areseparated by 4 foreign matter and the other contacts are closed, said capacitor completing a circuit through said device for application of said voltage across said other contacts when the contacts of both of said sets are held in open condition by foreign matter thereon.

4. In combination with a device adapted to be energized by the flow therethrough of electric current and means including two sets of switch contacts connected in series with each other and said device for applying to the latter Va voltage of a value suicient to energize the device when both sets of contacts vare closed, an impedance element connected in shunt around `a rst one of said sets of contacts and in series with the other set and having an impedance suliiciently higher Ain value than the impedance of said device to limit current flow through the device to a value insuticient to energize the device and to produce a voltage drop across said rst set of contacts substantially equal to the voltage of said source when said first contacts are separated by foreign matter and said second contacts are closed for application of said voltage to the device and said element in series, said impedance element completing a conductive path through said device for application of 'said voltage across said second contacts when the contacts of both of said sets are held in open condition by foreign matter thereon.

5. In combination with a device adapted to be energized by the flow therethrough of electric current of a predetermined value and means including more than one set of switch contacts connected in series with each other and said device for applying to the latter a voltage sufficient to ,energize Vthe device when all of the contacts are closed, individual shunting means for at lleast all except one ofsaid sets of contacts comprising impedance elements each having a value substantially higher `than the impedance of said .device to limit current ow through the latter to awvalue less than said predetermined value and to produce a voltage drop across Vthe Velement eqt'ial approximately to the full voltage of said source -Iwhen the element and the device are connected in series Cross the source, said voltage drop being applied across the set of contacts shunted by the element to breakdown foreign matter lseparating the contacts.V

6, In combination with a device adapted to be yenergized by the ow therethrough of electric current .of a predetermined value and means including more `than one set of switch contacts connected in series ywith each other and said device for applying to the latter a voltage suicient to energize the device when a 1 l of theco ntacts are closed, individual shunting Vmeans for at'rleast all except one of saidsets of contacts comprising impedance elements each having impedance.characteristics correlated with `such characteristics of said load Vto limit .current flow-V through the llatter to a `value 'less than said vpredetermined value and to produce-a voltage drop across the element equal substantially to full voltage of said source when theelement and the ydevice areconnected in series across the source, such voltage drop being vapplied across the set of contacts shunted by the element'to break down foreign matter separating the contacts.

References Cited in the'file of this patent UNITED STATES PATENTS 814,669 Boyce Mar. v13, 1906 2,183,838 Hornickel Dec. 19,. 1939 2,223,516 Goetz Dec. 3, 1940 2,528,558 'Sherman Nov. 7, 1950 2,701,965 Sherman Feb. 15,1955 

